When a water heater is installed, in most applications a drain pan must be installed underneath the heater tank to catch water which leaks or spills from the tank. Also, there must be a temperature and pressure relief valve ("T&P valve") installed on the water heater which releases water from the tank if excessive water temperature or pressure occurs. This frequently results in the plumber having to run two drain pipes to the exterior of the building, one for the drain pan and one for the T&P valve. The reason for requiring separate drain pipes is to prevent water released from the T&P valve from backing up into and overflowing the drain pan if the T&P valve should begin discharging at a high rate. Obviously, running two separate drain lines from a water heater to the exterior of the building may significantly increase the cost of labor and materials necessary to install the water heater.
The plumbing codes of some localities allow the drain pan outlet line and the T&P valve outlet line to be connected together if a swing check valve is installed to protect the drain pan from overflow in case a significant discharge from the T&P valve outlet. The use of the swing check valve allows a single drain pipe to be run to the building exterior, thereby saving plumbing costs. However, experience indicates that swing check valves frequently do not allow water to drain properly from the drain pan. This is because the swinging valve member is normally in the closed position. When the water level in the drain pan is high enough, it forces the swing member open just enough to allow the water to seep through. Because the drain pan is unpressurized, the only pressure acting to open the swinging valve member results from the hydrostatic head of the water in the drain pan. Typically, this pressure amounts to only a few inches of hydrostatic head. If a large amount of water is spilled into the drain pan, this small amount of pressure may be insufficient to open the swing member enough to allow the water to escape fast enough to prevent the water from spilling over the top of the pan. In such cases, the only option available to the plumber is to run two separate pipes to the exterior of the building.